Carbon is known to have five unique crystalline structures, including diamond, fullerene (0-D nano graphitic material), carbon nano-tube or carbon nano-fiber (1-D nano graphitic material), graphene (2-D nano graphitic material), and graphite (3-D graphitic material). The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall. Carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs) have a diameter on the order of a few nanometers to a few hundred nanometers. Their longitudinal, hollow structures impart unique mechanical, electrical and chemical properties to the material. The CNT or CNF is a one-dimensional nano carbon or 1-D nano graphite material.
The constituent graphene planes of a graphite crystallite in a natural or artificial graphite particle can be exfoliated and extracted or isolated to obtain individual graphene sheets of carbon atoms provided the inter-planar van der Waals forces can be overcome. An isolated, individual graphene sheet of carbon atoms is commonly referred to as single-layer graphene. A stack of multiple graphene planes bonded through van der Waals forces in the thickness direction with an inter-graphene plane spacing of approximately 0.3354 nm is commonly referred to as a multi-layer graphene. A multi-layer graphene platelet has up to 300 layers of graphene planes (<100 nm in thickness), but more typically up to 30 graphene planes (<10 nm in thickness), even more typically up to 20 graphene planes (<7 nm in thickness), and most typically up to 10 graphene planes (commonly referred to as few-layer graphene in scientific community). Single-layer graphene and multi-layer graphene or graphene oxide sheets are collectively called “nano graphene platelets” (NGPs). Graphene or graphene oxide sheets/platelets (collectively, NGPs) are a new class of carbon nano material (a 2-D nano carbon) that is distinct from the 0-D fullerene, the 1-D CNT, and the 3-D graphite.
Our research group pioneered the development of graphene materials and related production processes as early as 2002: (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A. Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006).
Isolated or separated graphene or graphene oxide sheets (NGPs) are typically obtained by intercalating natural graphite particles with a strong acid and/or oxidizing agent to obtain a graphite intercalation compound (GIC) or graphite oxide (GO), as illustrated in FIG. 5(A) (process flow chart) and FIG. 5(B) (schematic drawing). The presence of chemical species or functional groups in the interstitial spaces between graphene planes serves to increase the inter-graphene spacing (d002, as determined by X-ray diffraction), thereby significantly reducing the van der Waals forces that otherwise hold graphene planes together along the c-axis direction. The GIC or GO is most often produced by immersing natural graphite powder (20 in FIG. 5(A) and 100 in FIG. 5(B)) in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium perchlorate). The resulting GIC (22 or 102) is actually some type of graphite oxide (GO) particles. This GIC or GO is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion, which contains discrete and visually discernible graphite oxide particles dispersed in water. There are two processing routes to follow after this rinsing step:
Route 1 involves removing water from the suspension to obtain “expandable graphite,” which is essentially a mass of dried GIC or dried graphite oxide particles. Upon exposure of expandable graphite to a temperature in the range of typically 800-1,050° C. for approximately 30 seconds to 2 minutes, the GIC undergoes a rapid volume expansion by a factor of 30-300 to form “graphite worms” (24 or 104), which are each a collection of exfoliated, but largely un-separated graphite flakes that remain interconnected. A SEM image of graphite worms is presented in FIG. 6(A).
In Route 1A, these graphite worms (exfoliated graphite or “networks of interconnected/non-separated graphite flakes”) can be re-compressed to obtain flexible graphite sheets or foils (26 or 106) that typically have a thickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm). Alternatively, one may choose to use a low-intensity air mill or shearing machine to simply break up the graphite worms for the purpose of producing the so-called “expanded graphite flakes” (108) which contain mostly graphite flakes or platelets thicker than 100 nm (hence, not a nano material by definition).
Exfoliated graphite worms, expanded graphite flakes, and the recompressed mass of graphite worms (commonly referred to as flexible graphite sheet or flexible graphite foil) are all 3-D graphitic materials that are fundamentally different and patently distinct from either the 1-D nano carbon material (CNT or CNF) or the 2-D nano carbon material (graphene sheets or platelets, NGPs). Flexible graphite (FG) foils can be used as a heat spreader material, but exhibiting a maximum in-plane thermal conductivity of typically less than 500 W/mK (more typically <300 W/mK) and in-plane electrical conductivity no greater than 1,500 S/cm. These low conductivity values are a direct result of the many defects, wrinkled or folded graphite flakes, interruptions or gaps between graphite flakes, and non-parallel flakes (e.g. SEM image in FIG. 6(B), wherein many flakes are inclined at an angle deviating from the desired orientation direction by >30°). Many flakes are inclined with respect to one another at a very large angle (e.g. mis-orientation of 20-40 degrees). The average deviation angle is greater than 10°, more typically >20°, and often >30°.
In Route 1B, the exfoliated graphite is subjected to high-intensity mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, high-intensity air jet mill, or high-energy ball mill) to form separated single-layer and multi-layer graphene sheets (collectively called NGPs, 33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814. Single-layer graphene can be as thin as 0.34 nm, while multi-layer graphene can have a thickness up to 100 nm, but more typically less than 20 nm.
Route 2 entails ultrasonicating the graphite oxide suspension for the purpose of separating/isolating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation has been increased from 0.3354 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separated, isolated, or discrete graphene oxide (GO) sheets. These graphene oxide sheets can then be chemically or thermally reduced to obtain “reduced graphene oxides” (RGO) typically having an oxygen content of 0.001%-10% by weight, more typically 0.01%-5% by weight, most typically and preferably less than 2% by weight.
For the purpose of defining the claims of the instant application, graphene or NGPs include discrete (isolated or separated) sheets/platelets of single-layer and multi-layer pristine graphene, graphene oxide, or reduced graphene oxide (RGO). Pristine graphene has essentially 0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight. Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.
Isolated solid NGPs (i.e. discrete and separate sheets/platelets of pristine graphene, GO, and RGO having a typical length/width of 100 nm to 10 μm), when packed into a macroscopic-size film, membrane, or paper sheet (34 or 114 in FIG. 5(A) or FIG. 5(B)) by using, for instance, a paper-making process, typically do not exhibit a high thermal conductivity, as an example of useful physical properties. This is mainly ascribed to the notion that these sheets/platelets are typically poorly oriented and many types of defects are formed in the film/membrane/paper. Specifically, a paper-like structure or mat made from platelets of graphene, GO, or RGO (e.g. those paper sheets prepared by vacuum-assisted filtration process) exhibit many defects, wrinkled or folded graphene sheets, interruptions or gaps between platelets, and non-parallel platelets (e.g. SEM image in FIG. 7(B)), leading to relatively poor thermal conductivity, low electric conductivity, low dielectric breakdown strength, and low structural strength. These paper-like structures or aggregates of discrete NGP, GO or RGO platelets alone (without a resin binder) also have a tendency to get flaky, emitting conductive particles into air; but the presence of a binder resin significantly reduces the conductivity of the structure.
Graphene thin films (<5 nm and most typically <2 nm) can be prepared by catalytic chemical vapor deposition CVD of hydrocarbon gas (e.g. C2H4) on Ni or Cu surface. With Ni or Cu being the catalyst, carbon atoms obtained via decomposition of hydrocarbon gas molecules at 800-1,000° C. are deposited onto Ni or Cu foil surface to form a sheet of single-layer or few-layer graphene (2-5 layers in this case) that is poly-crystalline. These ultra-thin graphene films, being optically transparent and electrically conducting, are intended for applications such as the touch screen (to replace indium-tin oxide or ITO glass) or semiconductor devices (to replace silicon, Si). The Ni- or Cu-catalyzed CVD process does not lend itself to the deposition of more than 5-10 graphene planes (typically <5 nm and more typically <2 nm) beyond which the underlying Ni or Cu catalyst can no longer provide any catalytic effect. There has been no experimental evidence to indicate that CVD graphene film thicker than 5 nm is possible. Furthermore, the CVD process is known to be extremely expensive.
From semiconductor physics perspectives, on the one hand, multi-layer graphene sheets are a metallic or conductor material and single-layer graphene sheets are a semi-metal. The single-layer pristine graphene lacks an energy band gap because its valence and conduction bands touch each other and, hence, it is labeled as a semimetal. (In contrast, Si, a semiconductor, has an energy band gap of 1.1 eV between the conduction band and valence band of its electronic configuration.) The lack of a band gap limits usage of graphene in contemporary electronic devices. The band structure of single-layer graphene can be modified to open the band gap by many strategies, e.g., halogenation, oxidation, hydrogenation or noncovalent attachment of various molecules and species.
On the other hand, heavily oxidized graphene or graphene oxide (GO) is considered an insulating material and presumably can be used as a dielectric. However, the low thermal stability of GO (against heat exposure) reduces its dielectric resistivity, which is a drawback since thermal processing steps are often used during electronic device fabrication. Furthermore, multi-layer pristine graphene (>3 layers) and reduced graphene oxide (RGO) are essentially a conductor that cannot be used as a dielectric material or an insulating material.
Dielectric materials have attracted great attention because of their potential application in gate dielectrics, dynamic random access memory, artificial muscles and energy storage devices. Dielectric (ceramic) capacitors for energy storage suffer from poor processability (e.g. processing temperature usually exceeds 1,000° C.), high density, and low breakdown strength. Traditional high dielectric perovskite ceramics, such as barium titanate-containing composites, cannot be used at situations where diverse shapes are required. Compared with inorganic ceramics, polymers are more applicable in higher electric fields. Additionally, polymers possess the following advantages over inorganic ceramics: low weight, low cost, ease in processing, and self-healing. However, low operation temperatures restrict the further development of polymer dielectrics. Commercial capacitors are only used in limited applications such as cell phones, video/audio systems, and personal computers. For example, biaxial oriented polypropylene polymer-based capacitors can only be operated at temperatures below 105° C. Thus, materials with high dielectric constants, especially those which can be used in high temperature environments, have great potential in device applications.
With these drawbacks of current dielectric materials in mind, we proceeded to investigate the potential of using graphene-derived materials for dielectric applications. After an in-depth and extensive study, we have surprisingly discovered that halogenated graphene materials thicker than 10 nm are a good dielectric material. Halogenated graphene is a group of graphene derivatives, in which some carbon atoms are covalently linked with halogen atoms. The carbon atoms linked with halogens have sp3 hybridization and other carbon atoms have sp2 hybridization. This implies that halogenated graphene (also referred to as graphene halide) potentially can be an insulating material. For this purpose, thicker graphene halide films (>10 nm, preferably >100 nm, further preferably >1 μm, and more preferably >10 μm) are desired. However, although ultra-thin films (e.g. <<10 nm) of graphene fluoride have been produced by the catalytic CVD preparation of pristine graphene, followed by fluorination, thicker graphene fluoride films having a combination of desired physical and chemical properties have not been available. It is known in the art that thicker dielectric materials (thicker than 5-10 nm) tend to have low dielectric breakdown strength even though most of the current devices demand to have thicker dielectric components.
Thus, it is an object of the present invention to provide a cost-effective process for producing thicker films of graphene-derived materials that exhibit a high dielectric breakdown strength, high dielectric constants, adequate mechanical strength, good thermal stability, and good chemical stability.